The proposed research will attempt to: 1. define the physical and/or structural changes which occur in elementary bodies following phagocytosis by susceptible and non-susceptible cells; 2. define the biochemical events associated with the early phase of the growth cycle; 3. determine the time course and nature of the chlamydial mRNA and protein molecules synthesized throughout the growth cycle; 4. accurately define the temporal sequence of chlamydia DNA replication in the infected cell; 5. analyze replicative forms of chlamydial DNA and to obtain replicative DNA polymerases. We plan to use a variety of methods including electron microscopy, density gradient centrifugation, and molecular hybridization.